The present invention relates to a semiconductor laser and more particularly, a high-power semiconductor laser employed as a light source for a pickup in an optical disk drive for writing and reading of data on a minidisk, photomagnetic disk, CD-R, or the like.
High-power semiconductor lasers have been developed for producing an output power of more than 30 mW (or over 40 mW for CD-R) and used as light sources in disk drives for minidisks, photomagnetic disks, or CD-Rs. In each case, a high intensity of light output is needed for carrying out a data writing action while a low output power is sufficient for executing a data reading action.
Such high-power semiconductor lasers should provide substantial characteristics including (1) single lateral mode oscillation prior to emission of a high output power, (2) low astigmatic difference, (3) low ellipticity, (4) low noise, and (5) high operational reliability.
For implementation of (1) single lateral mode oscillation prior to emission of a high output power, a refractive index waveguide structure is employed where a light wave is confined in parallel to an active layer (referred to as in a lateral plane hereinafter). Also, a gain waveguide type of the semiconductor laser having no difference of the index of refraction on the lateral plane may be used. Although longitudinal multi mode oscillation in the gain waveguide type allows a lower optical feedback noise, it creates a high astigmatic difference. In addition, it fails to confine light in the lateral plane, action in the lateral mode will be unstable causing a kink. The semiconductor laser having the refractive index waveguide structure performs multiplexing in the longitudinal mode by superimposing a high frequency current of several hundreds megahertzes over a laser driving current in order to attenuate coherent radiation and thus, lower the optical feedback noise. However, this requires a high frequency superimposing circuit in addition to a basic semiconductor laser driving circuit, thus increasing the overall size, the consumption of power, and the production cost. Also, it is likely to generate a high frequency electromagnetic noise which may give serious damage to a relevant machine such as a computer.
It is also known to use a self-excited oscillation technique for shifting the longitudinal mode to the multi mode in the refractive index wavelength type of the semiconductor laser without performing the high frequency superimposing. The self-excited oscillation in the refractive index waveguide structure is implemented by decreasing the fraction index difference in the lateral plane (See "Semiconductor layer" by Ryoichi Ito et al, p. 121, Baifukan, 1989). When the refractive index difference in the lateral plane is decreased, light runs off laterally and out-of-stripe regions of the active layer which remain deenergized with no current being applied act as saturable absorption areas.
FIG. 7 illustrates an example of the semiconductor laser having a known refractive index waveguide structure. As shown, there are provided a GaAs substrate 1 of n-type (expressed by n-hereinafter), an n-Al.sub.x Ga.sub.1-x As cladding layer 2, an Al.sub.y Ga.sub.y-1 As active layer 3, a first Al.sub.x Ga.sub.1-x As cladding layer 4 of p-type (expressed by p-hereinafter), an n-GaAs current blocking layer 5b, a second p-Al.sub.x Ga.sub.1-x As cladding layer 6, a p-GaAs contact layer 7, and two, upper and lower, electrodes 8, 9. The n-GaAs current blocking layer 5b confines an injection current in an active area of a stripe shape having a width W and absorbs light produced in the active layer so that the complex refractive index is different between inside and outside of the stripe active area. This allows a light wave to be confined in the lateral plane and guided steadily in the stripe active area of width W.
As compared with the semiconductor laser with the absorption loss associated refractive index waveguide structure, shown in FIG. 7, where the current layer is made of GaAs, another refractive index waveguide structure having the current blocking layer made of AlGaAs is also known (Proceeding 17a-V-1 of 1992 Applied Physics Convention (Autumn) in Japan). Shown in FIG. 8 are an n-GaAs substrate 1, an n-Al.sub.x Ga.sub.1-x As cladding layer 2, an Al.sub.y Ga.sub.1-y As active layer 3, a first p-Al.sub.x Ga.sub.1-x As cladding layer 4, an n-Al.sub.z Ga.sub.1-z As current blocking layer 5a (z.gtoreq.x), a second p-Al.sub.x Ga.sub.1-x As cladding layer 6, a p-GaAs contact layer 7, and two, upper and lower, electrodes 8, 9. The mixed crystal factor z of Al in the n-Al.sub.z Ga.sub.1-z As current blocking layer 5a is greater than the mixed crystal factor x of Al in the cladding layers 2, 4 and 6 so that its refractive index is lower than that of the stripe recess. A difference in the refractive index causes a light wave to be confined to the stripe as guided. In the structure, absorption loss caused by the current blocking layer is avoided and laser oscillation will be implemented with efficiency for producing a higher output power. Also, when the mixed crystal factor z of Al is approximated to x of the cladding layer, the refractive index difference in the lateral plane decreases. Resultant escape of light from the stripe will trigger the self-excited oscillation. Accordingly, the semiconductor laser with the foregoing structure will minimize optical feedback noise (See Japanese Patent Laid-open Publication 5-160503 (1993)).
In general, the refractive index waveguide type semiconductor laser is directed to produce single longitudinal mode oscillation thus being high in the coherency. Hence, the optical feedback noise will be unavoidable failing to satisfy the requirement (4) of low noise. More specifically, when the return light reflected on a disk comes into the semiconductor laser, it causes the laser oscillation to be unstable. Such an optical feedback noise is not negligible in the reading of data as it may cause data reading error.
It is a good idea for minimizing the refractive index difference in the lateral plane in the structure shown in FIG. 7 to increase the thickness of the first p-Al.sub.x Ga.sub.1-x As cladding layer 4. However, the increased thickness allows the injection current to be hardly applied in a width W to the active layer. Consequently, the structure will be declined in the characteristics, e.g. a threshold oscillation is increased. The light escaped from the stripe is absorbed in the n-GaAs current blocking layer 5 and the self-excited oscillation will hardly be commenced. It is thus unfavorable to fabricate a high-power self-excited laser from the structure shown in FIG. 7. Such high-power self-excited lasers may be fabricated but with very low productivity and their mass production will never be practical.
When the refractive index difference type waveguide structure shown in FIG. 8 is decreased in the refractive index difference in the lateral plane, it may be similar in the characteristics to a gain waveguide type semiconductor laser. In particular, the astigmatic difference will be highly increased during irradiation of a low output. Also, the pattern of light emission extending in the lateral plane may be varied depending on an intensity of output power.
Although the above mentioned structure is also high in the slope efficiency (a ratio between output power and injection current) which is substantial for optimum oscillation thus minimizing an operating current, noise from its power source causes the output power to be fluctuated. This may result in generation of critical noise or internal fracture by excessive emission of light.